1. Field of Invention
The present invention relates to a protective coating applied to a sliding member and the like for the purpose of enhancing the wear resistance and decreasing the friction coefficient. More particularly, the present invention relates to hard carbon film as the protective coating and a method for producing the hard carbon film. Furthermore, the present invention relates to mechanical parts coated with an amorphous hard carbon film, used for automobiles and household electric appliances. In the present invention, the term xe2x80x9chardxe2x80x9d is used in accordance with the general terminology of tribology (c.f. for example xe2x80x9cTribologistxe2x80x9d (in Japanese) Vol. 44, No. 9, 1999, small special edition xe2x80x9chard materialsxe2x80x9d). Specifically, a sliding member harder than the opposite member, can be said to fulfill the hard property. Particularly, the hardness of hard film is Hv (Micro-Vickers Hardness Number) 1000 or more, more particularly Hv 1500 or more. Several examples of the amorphous hard carbon film applications are reviewed in Tribologist Vol. 41, No. 9, 1996, pages 760-771.
2. Description of Related Art
The conventional surface-treatment methods of metallic material for enhancing the wear resistance and scuffing resistance include nitriding, PVD (physical vapor deposition) and CVD (chemical vapor deposition). Metal carbide, e.g., TiC, or metal nitride, e.g., TiN, has been applied by the latter two methods as a hard film on the surface of the metallic material of a tool, metal die, and the like. Since these coating layers are as hard as Hv 2000-3000, but the friction coefficient lies in the range of from approximately 0.2 to 0.8 and is relatively high, sliding resistance with the opposite material increases according to circumstance. As a result, there arises problems of wear of the coating layer and damage of the opposite material.
The sliding members of the internal combustion engine of an automobile are exposed to severe sliding conditions, particularly an outer peripheral surface of a piston ring. A vane of a compressor and a plunger (piston) of a fuel injection pump are exposed to severe sliding conditions, too. The wear resistance required for these parts is, therefore, at a high level.
Heretofore, in order to ensure satisfactory sliding properties, particularly the wear resistance of these parts, high-grade wear-resistant materials are used and subjected to surface hardening treatment, such as nitriding and carburizing. However, the conventional method involves a problem in that it uses expensive material. In addition, since the surface hardening treatment must be carried out at high temperature of from several hundred degree C. to 1000xc2x0 C., dimensional accuracy is lowered due to thermal deformation and the hardness of the substrate (work piece) material is also lowered.
The plunger of a fuel injection pump is operated under a poor lubricating gasoline environment. A composite plating with dispersed hard particles is, therefore, applied on a surface of the plunger. The heat treatment must be carried out at approximately 400xc2x0 C. to enhance the hardness of the composite to the required level. High-grade material equivalent to SKD 11 (Cold Work Tool Steel) should, therefore, be used so as to avoid softening during the heat treatment. Under such circumstances, it has been desired to develop a method for forming wear resistant hard film at low temperature.
The amorphous hard carbon film formed by the CVD method with the aid of plasma or ion beam has high hardness in the range of from approximately 2000 to 3000 Hv. This film has attracted attention as the material of highly wear resistant coating. The amorphous hard carbon film may be referred to as a diamond-like carbon film, i-carbon film, hydrogenated amorphous carbon film (a-C:H) and the like. Each of these films mainly comprises amorphous carbon.
Although applications of such amorphous hard carbon film to sliding members have been considered, the inherently large compressive stress in the film reduces adhesion of the film to the substrate material and makes a thick coating of the film impossible.
It has been proposed to enhance the adhesion of the amorphous hard carbon film to the substrate by means of sandwiching between the substrate and the film an intermediate layer consisting of metal, metal nitride and metal carbide. For example, Japanese Examined Patent Publication No. 5-82472 discloses the use of carbide, carbo-nitride, oxy-carbide, carbo-oxy-nitride and carbo-boride of 4a, 5a and 6a family metal of Periodic Table, as well as carbide or carbo-nitride of Si as the intermediate layer. Japanese Unexamined Publication No. 10-130865 discloses an intermediate layer, which mainly comprises at least one metal element or its oxide, nitride or carbide selected from the group consisting of Al, Cr, Sn, Co. However, these proposals to improve the adhesion are connected with the intermediate layers rather than the surface layers which are directly involved in the sliding.
Surface Coating Technology, 47, 710-721 (1991) and Japanese Unexamined Patent Publication No. 3-240957 (Japanese Patent No. 2971928) discloses that low friction coefficient is obtained due to the adsorption of environmental gas to the silicon oxide (SiO2) formed on the sliding surface of an amorphous hard carbon-hydrogen-silicon film. Note that this silicon oxide is formed on the surface of the hydrogenated amorphous carbon film, which preliminarily contains Si in the film, during its sliding with the opposite member. In other words, silicon oxide is not present in the film from the beginning. The friction coefficient is, therefore, high at the beginning of sliding, and a prolonged time is necessary to attain a low level of friction coefficient.
It is, therefore, an object of the present invention to overcome the problems involved in the prior art and to provide an amorphous hard carbon film having high hardness low friction coefficient and good adhesion.
It is an another object of the present invention to provide mechanical parts, which are coated with an amorphous hard carbon film containing metal oxide on the sliding surfaces.
It is a further object of the present invention to provide an amorphous hard carbon film which contains metal oxide and which attains stably low friction coefficient from the beginning of sliding. Incidentally, the conventional amorphous carbon film cannot exhibit such friction coefficient from the beginning.
It is yet an object of the present invention to provide a method for producing the amorphous hard carbon film having the properties described above. A method for forming the amorphous hard carbon film on sliding surface of the mechanical parts according to the present invention should be such that high temperature is unnecessary.
The amorphous hard carbon film according to the present invention mainly comprises carbon and hydrogen and is characterized by containing metal oxide in the film. The metal oxide may be an oxide of at least one element selected from the group consisting of Si, Ti, B and W. The content of oxygen in the film is preferably from approximately 0.1 to 10 atomic %.
The amorphous carbon film, which comprises mainly carbon hydrogen and metal oxide, can be formed by means of introducing carbon material, metal-containing material and oxygen into a vacuum chamber, in which a substrate is placed.
The amorphous carbon film according to the present invention exhibits high hardness, improved wear resistance, and low friction coefficient. The amorphous carbon film according to the present invention can, therefore, be applied to the mechanical parts which have a sliding portion and are operated under conditions where lubrication is difficult.
The minor components of the amorphous hard carbon film according to the present invention are mainly oxide and contain fluorine, bromine, chlorine and the like contained in the starting materials in addition to a small amount of oxygen not combined as the oxide, and metallic elements.
In the present invention, the main components, i.e., carbon and hydrogen, form an amorphous structure detected by the Ar-laser Raman spectroscopy. Excellent sliding property is attributable to the amorphous structure. The metal oxide may be crystalline or amorphous.
In the present invention, the opposite member is a cylinder or cylinder liner made of cast iron or aluminum alloy in the case where the sliding member is a piston ring. The opposite members are a rotor or housing made of aluminum or iron for a vane of compressor, and a cylinder made of material equivalent to SKD 11 for a plunger of the fuel injection pump. These are non-limiting examples, and the amorphous hard carbon film according to the present invention can be embodied as any member, provided that the sliding properties of such film can be utilized.
The hardness of the amorphous hard carbon film according to the present invention is fundamentally determined by the hydrogen content of the film. When the hardness is Hv 1800 or less, the wear resistance is poor. On the other hand, when the hardness is Hv 2500 or more, the film is brittle. The hardness is, therefore, preferably from Hv 1800 to 2500. More preferably, the hardness is from Hv 1900 to 2400. When the film is thinner than 2 xcexcm, the wear resistance is unsatisfactory. On the other hand, when the film is thicker than 15 xcexcm, the film is liable to peel off under stress. The film is, therefore, preferably from 5 to 10 xcexcm thick.
Hereafter, the portions of a sliding member, on which the amorphous high carbon film is formed, are described. In the case of a piston ring, the film is formed on at least the outer peripheral surface. It is applied for the entire surface of the side rail and the ear portion of the spacer expander for a three piece type oil control piston ring, too. In the case of a vane of a compressor, the film is formed on one or more of the round top surface and side surface of the vane. In the case of a plunger of a fuel injection pump, the film is formed on at least the outer peripheral surface of the plunger. The substrate material of a piston ring, a vane, a plunger and the like may be made of conventional materials. The amorphous hard carbon film, which contains metal oxide according to the present invention, may be directly applied on the underlying metal or may be formed on such films as a nitriding layer, a Cr plating film, an Nixe2x80x94Coxe2x80x94P composite plating film, in which hard particles, e.g., silicon-nitride particles, are dispersed, and an ion-plating film of CrN, TiN and the like.
The amorphous carbon, which mainly comprises carbon and hydrogen and which contains metal oxide, can be formed by means of introducing carbon material, metal-containing material and oxygen into a vacuum chamber, in which a sliding member is placed. The film-forming technique may be the RF plasma enhanced CVD method, the ion-beam evaporating method, and the vacuum-arc method. An example of the RF plasma enhanced CVD method is hereinafter described.
Hydrocarbon gases such as methane, acetylene and the like can be used as the carbon material. Tetramethylsilane, tetraethylsilane, tetraethoxysilane, tetramethoxysilane, triethoxyboron, boron fluoride, tetra-i-propoxy titane, hexafluoride tungsten and the like can be used as the metal-containing materials.
Incidentally, the sliding member should undergo no heating during the film formation. Although the temperature of sliding member is elevated while being exposed to plasma, its temperature is kept 200xc2x0 C. or less.